In the manufacture of cavity resonators, periodic propagating structures and other waveguide components for microwave tubes of various types, or for other microwave devices, the copper usually chosen to ensure thermal conductivity is too good an RF electrical conductor for some purposes. This is the case when the cavity Q desired for optimum performance is much lower than the Q obtained with copper alone, or when the RF loss along a periodic structure or waveguide needed for device stability or to create a termination or mode filter is much grater than copper alone would provide. It accordingly becomes necessary to modify the cavity or waveguide, so that Rf power will be absorbed to a much greater degree than without the modification. In a coupled-cavity TWT, for example, it can be necessary to reduce cavity Q by a factor of several hundred at about 3 GHz, or by a factor of 40 to 80 at about 10 GHz.
A parameter called loss index is defined as the factor by which the Q of a cavity would be divided if the entire copper surface were coated. Loss index therefore is the same as the coating's effective surface resistivity in ohms per square, normalized to that of copper at the same frequency. Since the surface resistivity of copper itself varies with frequency, any change in loss index with frequency would be superposed on the change for bare copper to arrive at the coating's net surface resistivity at another frequency. As defined, the loss index of smooth, solid metals and metal alloys, in a layer thicker than a skin depth, should not vary with frequency, but it might well do so for porous or composite materials or very thin films. When it's not practical to coat the entire copper cavity interior, then the coating's loss index must exceed the factor by which the cavity Q is required to be divided.
To date a very popular coating has been a flame-or arc-sprayed layer of the alloy Kanthal A-1, fed into the flame or arc as a rod or wire. The main constituents of Kanthal A-1 are iron and chromium, in about the same proportion as in Type 430 stainless steel, and aluminum. Because of the aluminum oxide formed in the spraying of molten Kanthal, adhesion to the copper can be uncertain. The best chances for adhesion are when an undercoat of dendritic copper is applied in a prior operation. Recent research has quantified the factors contributing to the high loss index of this coating. (See: A. Karp, Microwave Physics of Flame-Sprayed Kanthal and Other Circuit Loss Coatings, 1985 IEDM TEchnical Digest, IEEE, pp. 354-6.) The primary factor is the alloy's high intrinsic resistivity (81 times that of copper for DC purposes but 9 times for RF purposes). The 2nd most important factor comes from the square root of the magnetic permeability that is effective at the frequency of interest (e.g., a multiplier factor measured to be 2.8 at 10 GHz). A 4th and rather minor factor is due to the roughness of the coating's surface. However, the third factor is non-trivial, and it comes from the fact that the coating is not solid, but an aggregation of metal blobs stuck together, it is believed, by ultrathin films of vitreous aluminum oxide fluxed with traces of the oxides of the other metals in the alloy (iron, chromium, cobalt).
Many of these factors can vary because of differences in operator technique. The coating thickness, which is large enough to weigh in cavity design, is also subject to variation. The highest observed loss indices for such coatings is around 50-55, which is sufficient for many past applications, but inadequate for many others and for some future ones.
Another commonly used coating is a sintered-on layer of iron spherules. This can be made quite thin, as is required for millimeter-wave applications, because the spherules are only a few microns in diameter, which is the result of preparing them via the Carbonyl process. However the intrinsic resistivity of iron is only 5.6 times that of copper for DC purposes or 2.37 times for RF purposes. The effective magnetic permeability, even at microwave frequencies, should be relatively high, but carbon impurity resulting from the Carbonyl process is detrimental to this permeability. The sponginess of the sintered-spherule layer is important to the overall loss index, but the layer becomes more compacted each time the assembly gets hydrogen fired in the course of tube construction. The nature of iron makes this unavoidable for both anhydrous (dry) and humidified (wet) hydrogen firing. The coating can also act as a wick in proximity to any molten braze alloy; when this occurs, loss is no longer obtained. Certain additives are helpful in preventing these problems but incur processing costs. Overall loss indices ranging from 15 to 23 have been measured at 10 GHz. This might be sufficient for some millimeter-wave requirements provided the index didn't decrease in the course of tube manufacture; other millimeter-wave applications require higher loss.
Certain coatings are used to metallize ceramics. However these metallizing coatings cannot be considered as loss coatings. The metal is typically molybdenum, a nonmagnetic, highly conductive metal, and it is very finely ground for the purpose. These factors all help make the effective loss index quite low. When this kind of coating is fused onto ceramic, a composition gradient develops so that the coating is mostly glass at the ceramic interface and almost entirely metal at the opposite surface. The coating of the present invention, however, is fused onto a metal substrate whereby no such composition gradient develops. Moreover the ceramic-metallizing coatings are fused at over 1200.degree. C., which is too high a temperature for copper parts.
There are schemes for lowering cavity Q, the ultimate objective, that do not use wall coatings, but instead use chunks or buttons of high-loss dielectric. Whereas the wall coatings interact with RF wall currents (or RF H-fields near the wall), the dielectric inserts interact with RF E-fields well inside the cavity. The material is typically ceramic infiltrated with carbon or containing some very finely powdered silicon carbide. The inserts tend to be large, expensive and hard to cool. The Anisotropy and nonreproducibility may cause problems. However, they have to date been the only available route to achieving the very large Q reduction factors required for S- and C-band cavity tubes.